1. The Field of the Invention
The present invention relates to an apparatus for driving and controlling a load that operates on an electric power source, and in particular, to the apparatus that has the capability of not only reducing electric power consumed by the load but also protecting an overheat occurring in the load and/or the apparatus.
2. Related Art
An apparatus for driving and controlling a load (hereafter, referred to as a “load drive control apparatus”) has been used widely by various types of equipment. Such an apparatus is applied to, for example, a blower motor and a cooling fan motor, which are incorporated in an air conditioner mounted on an automobile. Both of the blower motor and cooling fan motor are loads to be driven and controlled by the load drive control apparatus.
In such a system, it is required that electric power unnecessarily consumed by the motors during their non-operating periods of time is suppressed as less as possible. To try to satisfy such a request, there is proposed a technique disclosed by a U.S. Pat. No. 6,232,758 B1. This publication teaches a load drive control apparatus that is able to switch its operations between a normal operation mode and a stand-by operation mode, without using a large-size relay and applying an on/off signal to an ignition switch.
In this load drive control apparatus, if foreign materials creep into the rotation unit of for example a blower motor, the motor's torque is sometimes forced to increase or the motor is sometimes locked. If such a situation happens in fact, current flows, in most cases, excessively through switching elements composing part of the load drive control apparatus and/or the blower motor, which results in a sharp increase in the temperature at those switching elements and/or the blower motor. Thus, it is preferable to have overheat protection/control means of limiting the motor drive on the basis of temperature detected by a temperature sensor placed at, for example, a position nearer to the switching elements.
FIGS. 1A and 1B show changes in the temperature detected from a configuration, in which both of the foregoing load drive control apparatus and the overheat protection/control means are simply combined with each other, which is in an overload operation. FIG. 1A shows changes in the temperature in cases where a transition to an overheat-protecting operation was followed by an automatic stop of a blower switch but a user kept ignoring the on-state of the blower switch. Meanwhile, FIG. 1B shows changes in the temperature in cases where, in such a case, a user who noticed the stop of the blower fan doubtfully turned off the blower switch and then turned it on again.
In FIGS. 1A and 1B, a drive command signal is a signal to command the rotation of a blower and is given by an ECU of an air conditioner when the blower switch is on. A drive signal is made into an on/off signal for the switching elements when a drive control signal performs both of the control of the number of rotations based on a drive command signal and the overheat protection control. The foregoing load drive control apparatus makes a transition from its normal operation mode to its stand-by operation mode when the blower switch was manipulated into its off-state to stop the issue of the drive command signal. In response to this transition, supplying power to the drive control circuit that is in charge of driving the switching elements is stopped as well.
In the case of FIG. 1A, since the blower switch is kept to its on-state, the normal mode continues, thus the power being supplied to the drive control circuit without rest. Hence, the drive control circuit is capable of continuously executing an overheat protection control, in which the detected temperature is subjected control that allows changes within a temperature width regulated by both a predetermined protective temperature T1 and a predetermined release temperature T2. Because there is thermal resistance between the switching sensors and the temperature sensor, a chip temperature within each switching element (that is, a junction temperature) becomes higher than a detected temperature by the temperature sensor. However, the switching elements, which were turned off once, is again subjected to its on-operation when the detected temperature reduces down to the release temperature T2, whereby the chip temperature is limited to temperature less than its guarantee temperature.
On the other hand, in the state shown in FIG. 1B, the blower switch is manually turned off at a certain timing during the overheat protection control, resulting in that the power to the drive control circuit is once shut down. Hence the drive control circuit is reset, which loses the information indicative of states of the overheat protection control carried out before the shutdown. It is therefore possible for the drive control circuit to turn on the switching elements, if the temperature detected when the re-supply of the power is started is below the protective temperature T1. However, the blower switch is manually turned on and off in sequence after the detected temperature reached the protective temperature T1, there will be caused a fear that the chip temperature is beyond a guarantee temperature of the chips (refer to a range shown by a reference “A” in FIG. 1B). The reason is that, conjointly with an overload state (for instance, a motor locked state) bringing about a sharp increase in current, the drive control circuit is obliged to turn on the switching elements that should originally be turned off due to a high detected temperature.
In order to prevent such an unwanted overheating situation, there can be provided some countermeasures, including 1) a decrease in the protective temperature T1, 2) to embed a temperature sensor within the switching elements to measure a chip temperature with precision, and 3) to dispose, into the drive control circuit, non-volatile means for maintaining information indicative of controlled states. However, the means according to 1) is disadvantageous in that unfavorable conditions, such as higher surrounding temperature, will lead to frequent stops of the blower fan under the overheat protecting operation even in the normal use, thus spoiling user's amenity. Further, means according to 2) and 3) are difficult to employ, because a greater increase in the production cost of the system is inevitable.